mnt: tune the root mount before mounting a root yard

Otherwise the root yard can be propagated into the host mount namespace
and remain there and criu will fail, because it will not be able to
remove the roots yard.

It occures if we give a shared mount as root to "criu restore" and
criu converts it into a slave mount.
Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1984,8 +1984,6 @@ static int do_mount_root(struct mount_info *mi)
 						mi->shared_id, mi->master_id))
 		return -1;
 
-	mi->mounted = true;
-
 	return 0;
 }
 
@@ -2003,9 +2001,11 @@ static int do_mount_one(struct mount_info *mi)
 
 	pr_debug("\tMounting %s @%s (%d)\n", mi->fstype->name, mi->mountpoint, mi->need_plugin);
 
-	if (!mi->parent)
-		ret = do_mount_root(mi);
-	else if (!mi->bind && !mi->need_plugin && !mi->external)
+	if (!mi->parent) {
+		/* do_mount_root() is called from populate_mnt_ns() */
+		mi->mounted = true;
+		ret = 0;
+	} else if (!mi->bind && !mi->need_plugin && !mi->external)
 		ret = do_new_mount(mi);
 	else
 		ret = do_bind_mount(mi);
@@ -2481,9 +2481,6 @@ static int populate_mnt_ns(struct mount_info *mis)
 	struct mount_info *pms;
 	struct ns_id *nsid;
 
-	if (prepare_roots_yard())
-		return -1;
-
 	pms = mnt_build_tree(mis);
 	if (!pms)
 		return -1;
@@ -2506,6 +2503,17 @@ static int populate_mnt_ns(struct mount_info *mis)
 	if (validate_mounts(mis, false))
 		return -1;
 
+	/*
+	 * Set properties for the root before mounting a root yard,
+	 * otherwise the root yard can be propagated into the host
+	 * mntns and remain there.
+	 */
+	if (do_mount_root(pms))
+		return -1;
+
+	if (prepare_roots_yard())
+		return -1;
+
 	return mnt_tree_for_each(pms, do_mount_one);
 }